1. Field of the Invention
The present invention relates in general to a fluid opacity sensor, and, in particular, is directed to a fiber optic fluid opacity sensor for measuring opacity of a fluid in a process line.
2. Description of the Related Art
Opacity monitors are known for measuring how much light is blocked by gases passing through a conduit or stack as taught in U.S. Pat. Nos. 4,583,859 and 4,381,153. This measurement of opacity for the gases is a measurement of the amount of particles or smoke in the gas.
Paint is composed of several major ingredients including titanium dioxide (TiO.sub.2), acrylic latex, and water. At various stages in paint processing, the relative concentration of TiO.sub.2 must be measured to support process control. TiO.sub.2 is optically very dense and its concentration in mixtures with water and latex can be determined by the measurement of the opacity of the mixture.
Opacity may be defined as the inverse of transparency, or EQU O=I.sub.o /I.sub.tr ( 1)
where:
O=Opacity, PA1 I.sub.o =Incident intensity, and PA1 I.sub.tr =Transmitted intensity
Early attempts to measure paint opacity in a process line used a single light source and detector inserted in a probe. This probe was inserted into a process line. The measurement of light transmission through a small gap where the paint was permitted to flow provided a measure of paint opacity or optical density. Difficulties with a single source/single detector approach included thermal drift of the detector, intensity variation of the source, and 60 Hz line noise pick-up in the processing electronics. All of these problems resulted in poor repeatability and large errors in measured opacity over extended periods of time.
Because of the foregoing problems associated with the prior art devices, it has become desirable to develop a fluid opacity sensor with an adequate signal-to-noise ratio while maximizing the optical path length so as to avoid clogging the sensor with fluid deposits. The term fluid as used herein is meant to include both gases and liquids. The fluid opacity sensor should provide compensation for light source variations as well as compensating for temperature drift. The fluid opacity sensor should also allow for periodic calibration with a minimum of effort and for the measurement of transmitted light intensity over a wide dynamic range.